This invention relates to a pressure sensor and, more particularly, to a pressure sensor for use in an automotive internal combustion engine in which a pressure transmitting liquid is filled between a pressure detection assembly and a flexible diaphragm.
FIG. 5 illustrates one example of a conventional pressure sensor for use in detecting gas pressure within a combustion chamber of an internal combustion engine. In FIG. 5, reference numeral 1 designates a stainless steel case to be attached to an engine (not shown), 2 designates a diaphragm attached to an inner or lower end of the case 1, 3 designates an electrically insulating base, 4 designates a mounting bed made of a ceramic material and welded to the case 1, and 5 designates a pressure sensing element bonded to the mounting bed 4. The pressure sensing element 5 is a silicon semiconductor strain gauge. A pressure transmitting liquid of silicone oil 10 having a high boiling point is filled between the pressure sensing element 5 and the flexible diaphragm 2.
Reference numeral 6 designates terminals attached to the base 3, 7 designates bonding wires connecting the terminals 6 to the pressure sensing element 5, 8 designates a plug thread-engaged with the case 1, and 9 designates output lines extending through the plug 8 and connected at one end to the terminals 6.
The pressure increase within the combustion chamber causes the diaphragm 2 to flex upward in FIG. 5 and the pressure of the silicone oil 10 is increased, which in turn is sensed by the pressure sensing element 5. Then the pressure sensing element 5 generates an electrical signal indicative of the pressure change and supplies it through the bonding wires 7, the terminals 6 and the output lines 9 to an unillustrated external circuit for controlling the ignition timing of the ignition plug in accordance with the combustion conditions within the combustion chamber.
During assembly of the conventional pressure sensor as described above, the base 3 on which the terminals 6 are attached and the mounting bed 4 on which the pressure sensing element 5 is bonded are securely attached to the inner surfaces of the substantially cup-shaped case 1. Thereafter, the pressure sensing element 5 and the terminals 6 are wire-bonded within the cup-shaped casing 1 to electrically connect them by the bonding wires 7. Therefore, the wire-bonding operation must be carried out within the relatively narrow and hard-to-access space defined in the case 1, making the assembling operation efficiency relatively poor.
Also, since the mounting bed 4 supporting the pressure sensing element 5 is welded to the case 1, the welding heat applied during the assembly causes the mounting bed 4 to distort and deform, which results in an undesirable effect on the pressure sensing element 5 which may result in detection errors.
Further, the pressure sensing element 5 in the conventional pressure sensor is subjected to an extremely large temperature difference such as 800.degree. C. between two major surfaces. During operation of the engine, the pressure sensing element 5 may be heated to an elevated temperature at a pressure receiving surface at which the high-temperature slicone oil 10 is in contact with the element 5, and the opposite major surface of the element 5 facing to the atmosphere is kept at a relatively low temperature. Therefore, it was very difficult to compensate for the temperature difference of the pressure sensing element 5.
Also, since the flexible diaphragm 2 is securely attached at its circumferential edge to the lower end or relatively sharp edge of the case 1, the diaphragm 2 is always and repeatedly bent at the same circumferential edge supported by the case 1 with a small radius of curvature when the diaphragm 2 flexes due to pressure. Therefore, the diaphragm is subjected to repeated bending and could finally be raptured.
Further, since the internal volume of a detection cavity in which the pressure transmitting silicone oil 10 is filled is relatively large, a large amount of the silicone oil 10 is needed and a thermal expansion of the oil 10 is large, so that the error in pressure sensing is generated. Thus, it is desired that the internal volume of the detection cavity can be significantly reduced by a simple measure. Also, it is desired that a measure for compensating for the thermal expansion of the pressure transmitting liquid within the detection cavity be provided for an improved accuracy of the pressure sensing.
The pressure transmitting oil 10 is sealingly filled within the detection cavity, but a pressure is often generated in the oil and it is impossible to relieve this pressure once the sensor is assembled. The pressure in the pressure transmitting oil 10 results in a bias in the flexible diaphragm 2.